Laboratory studies were conducted to examine the leaf surface, epicuticular wax content, and spread area of primisulfuron spray droplet with and without surfactant on leaf surface of barnyardgrass and green foxtail. Adaxial and abaxial leaf surfaces were examined using scanning electron microscopy and leaf wax was extracted and quantified. The spread of 1-μl droplets of distilled water, primisulfuron solution (without surfactant), primisulfuron solution with a nonionic low foam wetter/spreader adjuvant (0.25% v/v), and with an organosilicone wetting agent (0.1% v/v) was determined on the adaxial leaf surfaces of each of the weed species. Stomata and trichomes were present on adaxial and abaxial leaf surfaces in both species. Green foxtail had more stomata per unit area on the adaxial as compared to the abaxial leaf surface. Barnyardgrass had more stomata on the abaxial than on the adaxial leaf surface. There was no significant variation in the number of trichomes per unit leaf area of green foxtail, and the number of prickles per unit area of leaf was significantly higher in adaxial than the abaxial leaf surface, in both young and old leaves. In barnyardgrass, there were more trichomes on abaxial than adaxial leaf surface. The mean value of the wax content per unit of leaf area in barnyardgrass and green foxtail was 35.9 μg cm⁻² and 19.1 μg cm⁻², respectively. On both species primisulfuron with a nonionic surfactant had more spread area than that without a surfactant, and the spread was even greater with organosilicone wetting agent. The spread area of primisulfuron droplet was higher on the leaf surface of barnyardgrass than on green foxtail when surfactant was added.

Nomenclature: Primisulfuron; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; green foxtail, Setaria viridis (L.) Beauv.